The present invention relates to a liquid crystal projector apparatus capable of modulating light from a light source by means of a plurality of liquid crystal panels and projecting synthetic light transmitted or reflected by the liquid crystal panels as a color image, and particularly to a liquid crystal projector and an adjusting method thereof that prevent luminance variation from occurring in each frame on a display screen of such a liquid crystal display.
Television receivers and the like have been spread which form a liquid crystal projector with a light source such as a lamp and a plurality of liquid crystal panels or spatial light modulators to project a color image on a screen or the like.
Such a liquid crystal projector generally separates white light emitted from the lamp into three primary colors by means of a dichroic mirror, modulates light of each of the three primary colors by means of a liquid crystal panel, and thereafter combines the three pieces of light with each other by means of a dichroic prism or the like. Then the liquid crystal projector projects the combined light onto a screen or the like via a projection optical lens to thereby form a large screen.
FIG. 5 is a plan view of a configuration of a three-panel-type liquid crystal projector apparatus formed with one liquid crystal panel for each of the R, G, and B colors (hereinafter referred to simply as a liquid crystal projector).
The liquid crystal projector apparatus shown in FIG. 5 condenses light emitted from a lamp 21 via an IR/UV cutoff filter in a light source optical system 30, and thereafter performs color separation.
The light source optical system 30 comprises two microlens arrays 31a and 31b each having a set of microlenses, a polarization beam splitter 32 for aligning a plane of polarization of light, and a condenser lens 33.
A white luminous flux that has passed through the light source optical system 30 first enters a dichroic mirror 34 for transmitting the red light R. Then, the dichroic mirror 34 transmits the red light R and reflects the green light G and the blue light B. The red light R transmitted by the dichroic mirror 34 is changed in traveling direction by 90xc2x0, for example, by a mirror 35, and then guided to a liquid crystal panel 37R via a field lens 36R.
In the meantime, the green light G and the blue light B reflected by the dichroic mirror 34 are separated by a dichroic mirror 38 for transmitting blue light.
Specifically, the green light G is reflected and thereby changed in traveling direction by 90xc2x0, for example, and then guided to a liquid crystal panel 37G via a field lens 36G. The blue light B passes through the dichroic mirror 38, thus traveling in a straight line, and then guided to a liquid crystal panel 37B via a relay lens 39, a mirror 40, a relay lens 41, a mirror 42, and a field lens 36B.
After being subjected to spatial light modulation by the liquid crystal panels 37R, 37G, and 37B, the pieces of light of the RGB colors enter crossed dichroic prisms 43 as a light combining means to be spatially combined with each other. Specifically, the red light R is reflected by a reflection plane 43a and the blue light B is reflected by a reflection plane 43b in a direction of a projection lens 44. The green light G passes through the reflection planes 43a and 43b, whereby the pieces of light of the three colors are combined with each other on a single optical axis.
Then, the projection lens 44 projects a magnified color image onto a screen 45 hung on a wall, for example, or a flat screen formed as a rear projector.
The R, G, and B liquid crystal panels 37 employed in the liquid crystal projector as described above generally have a transparent opposite electrode Pf and pixel electrode Pu with liquid crystal intermediate between the opposite electrode Pf and the pixel electrode Pu, as shown in FIG. 6. A thin-film transistor TFT serving as a switching device is formed by semiconductor techniques in part of the pixel electrode Pu in a divided pixel unit.
A gate of each thin-film transistor TFT is connected to a gate bus line Lx formed in a stripe manner in a line direction. A source of the thin-film transistor TFT is connected to a source bus line Ly arranged in a direction orthogonal to the gate bus line Lx.
A drain of each thin-film transistor TFT is connected to a transparent pixel electrode Pu divided for each pixel. A liquid crystal capacitance Cis is formed between an opposite electrode Pf and the pixel electrode Pu with liquid crystal intermediate between the opposite electrode Pf and the pixel electrode Pu.
Generally, the source bus lines Ly are sequentially selected in a horizontal direction by an X drive circuit to supply a display signal for one line, and the gate bus lines Lx are sequentially selected in a vertical direction, whereby a signal voltage for each pixel at an intersection of a source bus line Ly and a gate bus line Lx is supplied via a thin-film transistor to charge the liquid crystal capacitance Cis. Display information is thus written to modulate light passing through each liquid crystal pixel and thereby generate an image.
Incidentally, an auxiliary capacitance Cs as indicated by a dotted line is often formed for the purpose of compensating for leakage current between the source and the drain electrodes.
In some cases, black stripes are provided to the opposite electrodes Pf to minimize leak of light passing through portions other than the pixel electrode portions.
As described above, the liquid crystal panel (hereinafter referred to as the LCD panel) comprises an X shift register for sequentially selecting the source bus lines in a line direction to supply a video signal to be written; and a Y shift register for selecting the gate bus lines Lx to take in the supplied signal sequentially in a horizontal direction. Thus, active matrix driving is performed so that the video signal is written to each pixel in dot sequence or line sequence and the written signal is retained by a capacitance C (Cis+Cs) for a period of one field.
The driving of each LCD panel by direct-current voltage tends to cause electrochemical reaction in liquid crystal material and alignment layer material and at their interface, which results in faulty display. Therefore, in order to prevent application of the DC voltage to each pixel electrode of the LCD panel, an image signal with a field cycle whose polarity is reversed between positive and negative with a Vcom voltage as its center DC level is supplied, as shown in FIG. 7.
Thus, the TFT type LCD panel employs a reversal driving method, in which the LCD panel is driven on the basis of an FRP signal for reversing signal polarity, and a display signal reversed in polarity at least in each field period is supplied to each pixel.
In the case of simple field reversal driving, it is difficult to perfectly balance the driving at the time of positive polarity and the driving at the time of negative polarity, and thus variation of transmitted light in each field generally results in a flicker occurring at half a frame frequency.
Hence, the signal to each LCD panel is reversed in each frame, and line-sequence reversal driving, which applies signals opposite to each other in polarity between adjacent lines, is performed to reduce variation of the luminance signal on each field screen.
Thus, when the Vcom voltage for setting the center direct-current level is set at an appropriate value for each LCD panel and line reversal for reversing polarity in each line is performed, luminance variation is reduced and thereby flicker is made less noticeable. However, when gray frames in which white and black are repeated in every two lines continue, a flicker or change in brightness in each scanning line still occurs because white is more noticeable than black.
Even in the case of dot reversal, which supplies a display signal reversed in polarity for each adjacent pixel in a horizontal direction, luminance variation remains, and therefore flicker is not reduced.
When each LCD panel is driven by an alternating-current signal that is reversed in polarity in each field or each line as described above, flicker should be reduced; however, when balance of reversed signals supplied to pixels is disturbed, luminance of the display screen is changed in each field, and thus the luminance variation is detected as flicker.
Therefore, the conventional reversal driving method adjusts the Vcom voltage shown in FIG. 7 for each LCD so as to minimize the luminance variation of a display image formed by combining images after light modulation by the LCDs with each other by means of a combining prism or the like.
However, it is difficult to completely eliminate the luminance variation in a field cycle or the luminance variation in each line as described above simply by precisely adjusting the Vcom voltage for each LCD panel and adjusting the balance of the writing signal waveform.
While various reasons are conceivable for this difficulty, there are structural problems including variation with each pixel in stray capacitance on a thin-film transistor, variation of gate electrode lines (for example odd-numbered lines and even-numbered lines are unbalanced), light leak caused by irradiating the TFT transistor with a high intensity light source, and various other factors.
Moreover, since the display screen of a recent liquid crystal projector in particular is increased in size, the detection limit of flicker is very high, and therefore flicker is strongly sensed by the human eye. The PAL system, in particular, has a low field frequency as compared with the NTSC system, and therefore makes flicker more noticeable.
Furthermore, with increasing intensity of the projection light source and hence increasing brightness of the screen, level of the detection limit of flicker is further raised, and therefore such a flicker as has been undetected on a conventional liquid crystal display screen has become a problem.
It is an object of the present invention to provide a liquid crystal projector that is highly effective when the level of the detection limit of flicker is high and an adjusting method for reducing screen variations, especially for a large-sized liquid crystal display apparatus.
In order to achieve the above object, according to a first aspect of the present invention, there is provided an adjusting method for a liquid crystal projector, the liquid crystal projector having a plurality of liquid crystal panels for forming a display frame by spatially combining pieces of light modulated by the liquid crystal panels with each other, the adjusting method comprising the steps of: driving each of the plurality of liquid crystal panels by an adjusting pattern signal whose polarity is reversed in each frame, and adjusting a direct-current level of the adjusting pattern signal to temporally average light projected by each of the liquid crystal panels; and thereafter adjusting a direct-current level of a driving signal of a part of the liquid crystal panels to temporally average luminance variation of a synthetic projection image modulated and spatially synthesized by the liquid crystal panels.
The liquid crystal panels can be arranged so as to transmit R color, G color, and B color, and the part of the liquid crystal panels is for the B color. The adjusting pattern signal can be a line-reversal alternating-current signal that is reversed in polarity in each horizontal line of the display screen or a dot-reversal alternating-current signal that is reversed in polarity in each pixel in a horizontal direction.
The driving signal of the G color liquid crystal panel can be formed such that the driving signal of the G color liquid crystal panel is opposite in polarity from driving signals applied to the B color and R color liquid crystal panels.
According to a second aspect of the present invention, there is provided a liquid crystal projector with a plurality of liquid crystal panels for forming a display frame by spatially combining pieces of light modulated by the liquid crystal panels with each other, the liquid crystal projector, including: a driving circuit for at least supplying each of the liquid crystal panels with a display signal whose polarity is alternately reversed in each line; and an adjusting means for adjusting a common voltage level of the display signal supplied to each of the liquid crystal panels by the driving circuit; wherein the common voltage levels of the plurality of liquid crystal panels except a part of the liquid crystal panels are adjusted so that luminance variation of light modulated in single-frame units by the liquid crystal panels is minimized; and only the common voltage level applied to the part of the liquid crystal panels is adjusted to a level such that luminance variation in a frame cycle of projection light synthesized by the plurality of liquid crystal panels is minimized.
The plurality of liquid crystal panels may be for R color, G color, and B color, and the part of the liquid crystal panels is for the B color or the R color, which is low in visibility.
Each of the LCD panels can be reversed in each frame, and driven by line reversal or dot reversal. Polarity of the display signal applied to the G color liquid crystal is opposite in phase to that of the signals applied to the R color and B color liquid crystals.
As described above, according to the present invention, the display apparatus using at least two or more LCD panels, supplying image signals by the reversal driving method, and forming a synthetic image adjusts luminance variation of each of the LCD panels during reversal driving to be minimized, and further adjusts luminance variation of light modulated by the LCD panels at a spatially different place. Therefore, the detection limit of flicker on a display screen can be made extremely low. It is thus possible to greatly contribute to reducing flicker especially on a large bright screen.